In gas turbine engines a seal is required in the circumferential gap between turbine shroud seal segments, and between nozzle guide vanes. A problem encountered on existing engines is the parasitic leakage of cooling and sealing air into the turbine annulus of a gas turbine. This can represent a significant performance loss to the engine through the air not doing useful work in the upstream turbine blade rows, and spoiling the aerodynamics of the blade rows immediately downstream.